In advanced semiconductor integrated circuits (ICs), a trench structure formed in a semiconductor substrate is widely used to achieve various objects. For example, the trench structure is used to form a deep trench capacitor whose capacitance increases with the increase of the longitudinal surface area of a dielectric so as to enlarge the integration of semiconductor ICs. Moreover, the trench structure is used to form a trench isolation for isolating semiconductor devices in semiconductor ICs so as to improve problems of conventional LOCOS process such as the formation of so-called bird's beaks which occupy a larger amount of the surface area of the substrate, the occurrence of a less planar surface, and so on. In addition, the trench structure is also used to form a double diffused MOS transistor (DMOS), wherein a MOS transistor is formed within a trench, for applying to high power ICs.
FIG. 1 is a cross-sectional view showing an example of a conventional trench structure. In FIG. 1, a trench 2 is formed on a semiconductor substrate 1 through the conventional photolithography and etching processes (not shown). Then, a polycrystalline silicon layer 3 is deposited on the surface of the semiconductor substrate 1 through the low pressure chemical vapor deposition (LPCVD) method so as to substantially fill the trench 2. However, when the trench 2 is filled with the polycrystalline silicon layer 3, in fact, at least one dimple 10 always occurs on the polycrystalline silicon layer 3 above the trench 2. As a result, the polycrystalline silicon layer 3 deposited on the trench 2 which is formed on the semiconductor substrate 1 has a non-planar surface.
During the sequential manufacturing steps of semiconductor ICs, the above-mentioned dimple phenomenon will cause several disadvantages such as the following:
(1) deterioration in step coverage of layers deposited on the polycrystalline silicon layer 3 such that it is difficult to perform the subsequent manufacturing steps of exposure and alignment, and the residue held in the dimple 10 after the subsequent etching process causes a problem of leakage or shortage so as to reduce the yield of the products; PA1 (2) difficult removing of particles held in the dimple 10 resulting in the defect which reduces the yield and the reliability of the products; and PA1 (3) easy holding of an unnecessary material such as the etching solvent, the cleaning solution, etc., which causes the reduction of the reliability of the products due to the dimple 10.